Intermetal dielectric layers are commonly used to isolate conducting structures, such as metal layers, from subsequently deposited conducting layers. Intermetal dielectric layers are also useful in performing a planarization function. A typical prior art process for forming an intermetal dielectric layer consists of depositing multiple layers of oxide over the underlying metal layer. For example, a layer of silicon dioxide first covers the metal layer, followed by a spin on glass layer, followed by a second layer of silicon dioxide. The multiple layers of oxide are then patterned and etched to form via holes through to the underlying metal layer.
However, it has been found that during the stripping of the photoresist layer used in masking the etching of the oxide layers, the spin on glass at the sidewall of the via hole is exposed to oxygen plasma. Therefore, the moisture absorption of spin on glass damaged by the oxygen plasma treatment can be several orders of magnitude higher than that of cured spin on glass. The oxygen plasma used to strip the photoresist contaminates and adversely deteriorates the inner sidewalls of the via holes. The term used for this contamination is "poisoned via". See "A Study of Plasma Treatments on Siloxane SOG", C. K. Wang et. al., at page 101, VMIC (1994).
Conventional methods to solve the poison via problem are exemplified by U.S. Pat. Nos. 5,643,407 to Chang and 5,665,657 to Lee. However, these approaches attempt to solve the problem by minimizing the effect of the oxygen plasma.
What is needed is a method for forming a planarized intermetal dielectric layer without contamination during the stripping of the photoresist.